feat: block reference counting

This patch prevents deleting of a block from a vault if the block has
files referring to it. It adds a ConstraintError that is thrown if this
is attempted.
Merge pull request #114 from painterjd/feat_no_delete_block_with_refs

feat: block reference counting
chore: misc formatting, comment fixes
Merge branch 'master' into enhancement_hook_storage_url

Conflicts:
	deuce/tests/test_sqlite_storage_driver.py